Add support for continuations plugin in Scala 2.11.

In Scala 2.11, the continuations plugin is released under different
artifact id and we have to depend on the library part separately.

Note that continuations plugin is published using wrong cross-versioning
scheme. See scala/scala-continuations#5 for details.

We'll need to upgrade the plugin dependency once that issue is resolved.
Merge pull request #26 from gkossakowski/scala-2.11

Add support for continuations plugin in Scala 2.11.